Various types of spark-ignition, compression-ignition, and combination internal combustion engines use direct injection of fuel into the combustion chamber to reduce fuel consumption and feedgas emissions. These may include direct-injection spark-ignition (DISI) engines fueled by gasoline or gasoline/alcohol mixtures, compression-ignition engines fueled by diesel fuel, or combination engines fueled by gasoline or other fuels that may operate in a spark-ignition mode and a compression-ignition mode, sometimes referred to as homogeneous charge compression ignition (HCCI) mode, for example. A high-voltage power supply may be provided to generate the current required for desired performance of the fuel injectors for these applications, with representative voltages in the range of 60V or more compared to the nominal battery voltage of 12V or 24V, for example.
Manufacturers continue to improve control of internal combustion engines to enhance fuel economy and performance while reducing feedgas emissions using more sophisticated sensing and processing hardware and software. To improve control of the combustion process, ionization current sensing (or ion sense) uses a bias voltage applied across a sensor positioned within the combustion chamber to generate a current signal indicative of the combustion quality and timing. The bias voltage for reliable ion current signals often exceeds the voltage available directly from the vehicle battery so that a boost circuit or high voltage power supply is used to provide a bias voltage in the range of 85V or more, for example. Some spark-ignition engines provide the high-voltage supply by switching the ignition coil or using the ignition coil to charge a capacitor during the spark generation and then discharge the capacitor to provide the bias voltage during the ion sense period. While suitable for some applications, these systems do not provide a bias voltage for ion sense when no spark is generated, such as during compression-ignition mode in HCCI engines, for example.